1. Field of the Invention
This application relates to fastener driving devices and more particularly to power operated fastener driving devices having a system for avoiding the problems associated with dry firing.
2. Description of Related Art
Fastener driving devices of the power operated variety can be either pneumatically operated, electrically operated or internal combustion operated. Such devices involve the expenditure of energy at a level which makes it important that each operation constitutes a desired operation for driving a fastener into a workpiece. Much of the available energy is used up in the driving action itself. Consequently, when the device is operated without a fastener in the drive track to be driven, all of the energy normally provided to effect the driving action must be absorbed by the piston hitting the rubber bumper at the end of the drive stroke of the fastener driver. Where a dry firing of this type takes place after each fastener supply provided by the magazine assembly is exhausted, considerable damage and wear to the bumper and intimately related components, as well as damage to the workpiece, can occur. In recognition of this damage possibility, many power operated fastener driving devices have been equipped with some sort of dry firing prevention mechanism.
One example of a dry firing prevention mechanism is disclosed in U.S. Pat. No. 6,012,622. Essentially, dry firing is prevented by sensing fastener depletion and locking out the contact trip assembly in its extended position in response to a depletion signal or movement by the sensor. While arrangements of this type effectively reduce dry fire related component damage, experience has shown that other areas of possible damage are brought into being. The contact trip element when in its extended position is intentionally located so that it is the first structure to be engaged with another structure by the user in normal operation. This location makes it susceptible to being either deliberately or accidentally engaged when the depletion system has locked the contact trip element in its extended position The result is that with lockout depletion sensing systems of the prior art, damage to the contact trip assembly sometimes takes place which would not otherwise take place in the absence of the lockout depletion sensing system. There is still a need for a depletion sensing system which can effectively protect not only components susceptible to dry fire damage, but the contact trip assembly as well.